I. Field of the Invention
This invention relates to a mobile radio system, and in particular to a radio repeater (cell enhancer) for a digital time-division-multiple-access (TDMA) mobile radio system.
II. Prior Art and other Considerations
In a mobile radio system, there often exist areas of poor coverage within the service area of a base station, where poor signal reception by the base station from a mobile (and vice versa) results in unacceptable voice quality, or even the total loss of a call. One solution to this problem is the provision of a wideband repeater or re-radiator. This however suffers from certain disadvantages, viz (i) it would re-radiate all signals within the frequency range covered-including unwanted signals, for example from other radio systems which may be using the band; (ii) intermodulation and wideband noise; and (iii) the power level of each channel within the band would be amplified by an equal amount regardless of the actual signal strength.
A radio repeater is known (see the specification of our co-pending EP patent application no. 0274857) which utilizes a plurality of frequency-agile single-channel amplifier units for amplifying only traffic-carrying channels. This repeater is an analogue frequency-division-multiple-access (FDMA) system, and so is not suitable for digital systems such as the proposed pan-European GSM (Groupe Speciale Mobile) mobile radio system.